1. Field of the Invention
The present invention relates to fluid infusion systems in general, and specifically to a rapid, venous infusion system which controls the percentage of the volume of a patient's intravascular blood space occupied by red blood cells known as the "hematocrit" and empirically monitors a patient's estimated blood volume by using physiological inputs provided manually by the operator or automatically by sensors.
2. Description of the Related Art
Every acutely-ill patient who has a depleted blood volume from trauma or other disease process will require infusion of intravenous (IV) fluids to help correct the resulting cardiovascular deterioration. When large volumes of IV fluids are needed to stabilize the circulatory system, a point is reached when transfusion with a red blood cell (RBC) source is required in order to prevent extreme hemodilution (reduction in concentration of RBC) in the patient. Transfused blood is a suspension composed of red blood cells, having a source hematocrit given by HCT.sub.S with the remainder consisting of plasma protein colloids and water.
Many types of surgery such as major vascular repair, orthopedic surgery, organ transplant, and trauma create transfusion needs related to continued replacement of an ongoing blood loss. The decisions of 1) when to transfuse blood and 2) how much blood to transfuse is usually based on factors such as the most recent hematocrit (HCT) value of the patient, size of the patient, and amount of crystalloid or colloid fluids needed to be administered. The timing and rate at which to infuse blood into a patient is often based on an "empirical" judgment of the attending medical staff. Such a trial-and-error approach frequently results in substantial variability of patient hematocrit (HCT) values because blood is traditionally transfused in a unit by unit manner when the hematocrit HCT falls below a "transfusion trigger". In massively transfused patients, it is important to maintain blood volume by blending packed red blood cells and dilution fluids in the proper proportion for regulation of the hematocrit (HCT).
Several methods are disclosed in the prior art to estimate a patient's blood volume using kilogram body weight or body surface area. Methods have recently been described to estimate a patient's blood volume using intravenously injected dilution markers. Computational models used to guide fluid administration during expansion of a patient's blood volume assumes that 1) approximately 30% of isotonic crystalloids infused will remain within the intravascular compartment, and 2) approximately 100% of colloids infused will remain within the intravascular compartment during an initial 30 minute time period. Approximately 25-30% of urine output comes from the plasma volume based on urine osmolarity.
Examples of apparatus and components for rapid fluid infusion have been described in U.S. Pat. Nos. 3,731,679; 3,990,444; 4,138;288; 4,178,927; 4,210,138; 4,217,993; 4,256,437; 4,705,508; 4,747,826; 4,874,359; 5,061,241; 5,385,539; 5,423,747; and 5,573,502. These devices employ a single infusion pump (usually a roller pump) to dispense fluid/blood mixtures from a cardiotomy reservoir through a heat exchanger and air trap and then to the patient. Regulation of a patient's hematocrit (HCT) is attempted by the operator who must add fluid or units of blood to the mixing chamber in a schedule that properly modulates the composition of the infusion delivered via the single pump. These one-pump infusion systems can be wasteful of a critical donor blood supply because extra units of blood are needed to "prime" the mixing reservoir. A blood infusion device which will achieve a pre-selected hematocrit (HCT) in the patient as large or small volumes of physiologic fluids and blood are rapidly administered to patients represents an advantage.
No blood infusion device now exists which can rapidly deliver liters of physiologic fluid and blood while regulating a patient's hematocrit HCT to attain a desired hematocrit value (HCT*), in addition to monitoring the patient's estimated blood volume.